This invention relates to a primer composition for silicone pressure-sensitive adhesives, and more particularly, to a primer composition for use in a pressure-sensitive adhesive tape for providing a firm bond between a plastic film and a silicone pressure-sensitive adhesive layer thereon.
Prior art heat resistant pressure-sensitive adhesive tape typically includes a plastic film backing such as polyester film or polyimide film on which a heat resistant, silicone pressure-sensitive adhesive is applied to form an adhesive layer. The pressure-sensitive adhesive tape is generally wound into a roll form. When the adhesive tape is forcedly unraveled on use, the silicone pressure-sensitive adhesive layer can sometimes be stripped from the backing. Also when the adhesive tape is cut, it sometimes happens that only the backing is torn off, but the silicone pressure-sensitive adhesive layer remains uncut and becomes stretched with strings being drawn. Sometimes the adhesive layer is partially separated apart from the backing.
Such problems arise on use of the pressure-sensitive adhesive tape because the adhesion between the plastic film backing and the silicone pressure-sensitive adhesive is not fully tight. One common solution employed heretofore is to apply a carbon functional silane-based primer composition to a film backing as an adhesion improver, then applying a silicone pressure-sensitive adhesive thereon, for thereby improving the adhesion between the film backing and the silicone pressure-sensitive adhesive. However, the adhesion achieved thereby was not satisfactory.
A number of primer compositions using organopolysiloxane have been proposed for helping silicone pressure-sensitive adhesives firmly bond to plastic films. For example, JP-B 54-44017 discloses a primer composition comprising a diorganopolysiloxane having hydroxyl groups at both ends of its molecular chain, an organohydrogenpolysiloxane, and a tin salt of organic carboxylic acid. JP-B 6-39584 discloses a primer composition comprising a diorganopolysiloxane having hydroxyl groups at both ends of its molecular chain, an organohydrogenpolysiloxane, a vinyl and/or acrylic copolymer containing hydrolyzable silyl groups, and a tin salt of organic carboxylic acid.
These primer compositions, however, sometimes fail to provide a satisfactory bond. For example, in the event of addition curing type silicone pressure-sensitive adhesives which are recently on use and to be cured at lower temperatures, it is more difficult to provide a firm bond between the pressure-sensitive adhesive and the backing. This is true especially when the backing is of polyimide film having better heat resistance. It is thus desired to have a primer composition capable of forming a tight bond therebetween.
Therefore, an object of the invention is to provide a primer composition capable of forming a tight bond between various film backings and silicone pressure-sensitive adhesives.
The invention pertains to a primer composition comprising an alkenyl group-containing organopolysiloxane, an organohydrogenpolysiloxane, a platinum base curing catalyst, and a retarder. It has been found that the objects are achieved when the alkenyl group-containing organopolysiloxane is a diorganopolysiloxane having an average degree of polymerization of 500 to 2,000 and alkenyl groups at both ends of its molecular chain in an amount of 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane. When various plastic film backings are treated with this primer composition, and silicone pressure-sensitive adhesives are applied thereto, very tight bonds are established between the backings and the adhesives. This is true even when the plastic film backings are polyimide films which are believed difficult to establish a tight bond with silicone pressure-sensitive adhesives, and also when the adhesives are addition curing type silicone pressure-sensitive adhesives. As a consequence, satisfactorily heat resistant pressure-sensitive adhesive tapes are obtained.
Accordingly, the invention provides a primer composition for silicone pressure-sensitive adhesives, comprising
(A) a diorganopolysiloxane having alkenyl groups at both ends of its molecular chain and an average degree of polymerization of 500 to 2,000, the amount of alkenyl groups being 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane,
(B) an organohydrogenpolysiloxane having at least two hydrogen atoms directly attached to silicon atoms in a molecule,
(C) a platinum base curing catalyst, and
(D) a retarder.
In one preferred embodiment, the primer composition further includes
(E) an organopolysiloxane having at least one epoxy group-containing organic group, at least one hydrogen atom, and at least one alkoxysilyl group-containing alkyl group, each directly attached to a silicon atom, in a molecule and/or
(F) at least one additive selected from among a fatty acid, an acid anhydride resulting from intermolecular condensation thereof, and an intramolecular acid anhydride thereof.
In the primer composition for silicone pressure-sensitive adhesives according to the invention, component (A) is a diorganopolysiloxane having alkenyl groups at both ends of its molecular chain and an average degree of polymerization of 500 to 2,000, the amount of alkenyl groups being 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane. The preferred diorganopolysiloxane is of the following general formula (1). 
Herein R1 is alkenyl, R2 is a substituted or unsubstituted monovalent hydrocarbon group, xe2x80x9caxe2x80x9d is an integer of 1 to 3, and n is from 500 to 2,000.
Preferably R1 stands for alkenyl groups of 2 to 8 carbon atoms, such as vinyl, allyl and butenyl, with vinyl being most preferred. R2 stands for substituted or unsubstituted monovalent hydrocarbon groups of 1 to 16 carbon atoms, especially 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl and octyl, cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl and allyl, aryl groups such as phenyl and tolyl, and aralkyl groups such as benzyl and phenethyl, and substituted groups in which some or all of the hydrogen atoms on the foregoing groups are replaced by halogen atoms, cyano groups or the like, for example, haloalkyl groups such as chloropropyl and trifluoropropyl. Of these, methyl and phenyl are preferred for the industrial use.
The diorganopolysiloxane having alkenyl groups at both ends of its molecular chain represented by formula (1) has an average degree of polymerization of 500 to 2,000 and alkenyl groups in an amount of 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane. Preferably, the average degree of polymerization is from 1,000 to 2,000 and the amount of alkenyl groups is from 0.001 to 0.0025 mol per 100 g of the diorganopolysiloxane. With an average degree of polymerization of less than 500, the primer composition in the cured state is likely to strip from the film surface, failing to exert its effects. An average degree of polymerization of at least 500 prevents stripping and ensures primer effects. With an average degree of polymerization of 1000 to 2,000, the primer composition forms a more tight bond. An average degree of polymerization of more than 2,000 aggravates the adhesion between the pressure-sensitive adhesive and the backing. If the molar amount of alkenyl groups is less than 0.001 mol per 100 g of the diorganopolysiloxane, the primer composition will not cure. In excess of 0.005 mol/100 g, the primer effects are restrained.
The organohydrogenpolysiloxane (B) serves as a crosslinking agent to component (A) such that hydrogen atoms directly attached to silicon atoms (SiH groups) in component (B) undergo hydrosilylation reaction with alkenyl groups in component (A) for curing. The organohydrogenpolysiloxane is not critical as long as it has at least two hydrogen atoms directly attached to silicon atoms (SiH groups) per molecule. Illustrative examples are represented by the following general formulas (2) to (4). 
In the formulas, R3 is a substituted or unsubstituted monovalent hydrocarbon group, b is an integer of at least 2, d is an integer of at least 3, each of c, e and f is an integer inclusive of 0.
Preferably, R3 stands for substituted or unsubstituted monovalent hydrocarbon groups of 1 to 16 carbon atoms, especially 1 to 8 carbon atoms, and free of aliphatic unsaturation, for example, alkyl groups such as methyl, ethyl, propyl, butyl and octyl, cycloalkyl groups such as cyclohexyl, aryl groups such as phenyl and tolyl, and aralkyl groups such as benzyl and phenethyl, as well as substituted groups in which some or all of the hydrogen atoms on the foregoing groups are replaced by halogen atoms, cyano groups or the like, for example, haloalkyl groups such as chloropropyl and trifluoropropyl. Of these, methyl and phenyl are preferred for the industrial use.
The organohydrogenpolysiloxane preferably has a viscosity of about 1 to 500 centipoise, and especially about 1 to 100 centipoise at 25xc2x0 C.
Components (A) and (B) are blended in such amounts that the ratio of the moles of hydrogen atoms directly attached to silicon atoms in the organohydrogenpolysiloxane to the moles of alkenyl groups in the both end alkenyl group-terminated organopolysiloxane, represented as H/Vi hereinafter, may range from 0.5/1 to 10/1, and especially from 1/1 to 4/1. With a H/Vi below 0.5, the primer composition will not fully cure. With a H/Vi beyond 10, the primer composition may degrade its adhesion with the lapse of time (referred to as aged adhesion).
On use, a platinum base curing catalyst (C) is added to the primer composition, which is cured by heating. The platinum base curing catalyst is preferably platinum or platinum base compounds. Exemplary platinum base compounds include platinum black, chloroplatinic acid, alcohol-modified chloroplatinic acid, and complexes of chloroplatinic acid with olefins, aldehydes, vinylsiloxanes and acetylene alcohols.
An appropriate amount of the platinum base curing catalyst is preferably about 1 to 500 parts, and especially about 5 to 100 parts by weight, calculated as platinum, per million parts by weight of the primer composition. Less than 1 ppm of platinum may fail to achieve the curing effects whereas more than 500 ppm may degrade the aged adhesion of the primer composition and is uneconomical.
Component (D) is a retarder for restraining the composition from reacting near room temperature. It may be selected from those compounds which are conventionally used in the addition reaction between an alkenyl-bearing polysiloxane and an organohydrogenpolysiloxane in the presence of a platinum base curing catalyst.
Useful reaction regulators include acetylene alcohols, vinyl-bearing siloxanes and silylated acetylene alcohols, as well as combinations of two or more reaction regulators as disclosed in JP-A 9-143371. Exemplary are 3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, 3-methyl-1-penten-3-ol, 1-ethynylcyclohexanol, 1,1,3,3-tetramethyl-1,3-divinyl-disiloxane, methylvinylcyclotetrasiloxane, and 3-methyl-3-trimethylsiloxy-1-pentyne.
An appropriate amount of the reaction regulator blended is 0.01 to 10 parts, and especially 0.1 to 5 parts by weight per 100 parts by weight of components (A) and (B) combined. Less than 0.01 part would be too small to control the reaction whereas more than 10 parts would adversely affect cure.
In one preferred embodiment, the primer composition further includes (E) an organopolysiloxane having at least one epoxy group-containing organic group, at least one hydrogen atom, and at least one alkoxysilyl group-containing alkyl group, each directly attached to a silicon atom, in a molecule. This component is effective for further enhancing adhesion.
Preferred as component (E) are those of the following general formulas (5) and (6). 
Herein, R3 is as defined above. R4 is hydrogen or R3. R5 is an epoxy group-containing organic group. R6 is alkoxysilyl group-containing alkyl group. Each of s, u, v, w, y and z is an integer of at least 1, and each of t and x is 0 or an integer of at least 1.
The epoxy group-containing organic groups are exemplified by the following. 
Herein R7 is an alkylene group of 1 to 6 carbon atoms, preferably 2 to 4 carbon atoms, which may be separated by an oxygen atom, or an oxyalkylene group. Specific examples of the epoxy group-containing organic group are given below. 
The alkoxysilyl group-containing alkyl groups are exemplified by the following. 
Herein R2 is as defined above, R8 is an alkylene group of 1 to 6 carbon atoms, preferably 2 to 4 carbon atoms, R9 is an alkyl group of 1 to 6 carbon atoms, and m is an integer of 1 to 3, preferably 2 or 3. Specific examples of the alkoxysilyl group-containing alkyl group are given below. 
Illustrative, non-limiting, examples of component (E) are given below. 
An appropriate amount of component (E) blended is 0.01 to 10 parts, and especially 0.1 to 1 part by weight per 100 parts by weight of components (A) and (B) combined. Less than 0.01 part of component (E) would be too small to enhance adhesion whereas more than 10 parts would adversely affect cure.
In another preferred embodiment, the primer composition further includes (F) at least one additive selected from among a fatty acid, an acid anhydride resulting from intermolecular condensation thereof, and an intramolecular acid anhydride thereof. This organic compound is also effective for enhancing the adhesion of the primer composition.
Exemplary additives are those of the following formulas (7) and (8). 
In the above formulas, R10 is hydrogen or a saturated or unsaturated, substituted or unsubstituted, p-valent hydrocarbon group of 1 to 30 carbon atoms, and preferably 5 to 20 carbon atoms, and p is an integer of 1 to 10, and preferably 1 to 5. R11 is a saturated or unsaturated, substituted or unsubstituted, 2q-valent hydrocarbon group of 1 to 30 carbon atoms, and preferably 2 to 20 carbon atoms, and q is an integer of 1 to 3.
The hydrocarbon groups represented by R10 and R11 include alkyl groups, aryl groups such as phenyl, aralkyl groups, combinations of these groups; divalent or more valent hydrocarbon groups in which a predetermined number of hydrogen atoms are eliminated from the foregoing monovalent hydrocarbon groups; substituted hydrocarbon groups in which hydrogen atoms on the foregoing hydrocarbon groups are replaced by halogen atoms or hydroxyl groups; and substituted hydrocarbon groups in which two hydrogen atoms attached to carbon atoms in an alkyl group are replaced by an oxygen atom, that is, having Cxe2x95x90O.
Illustrative, non-limiting, examples of component (F) are given below. 
An appropriate amount of component (F) blended is 0.01 to 10 parts, and especially 0.1 to 1 part by weight per 100 parts by weight of components (A) and (B) combined. Less than 0.01 part of component (F) would be too small to enhance adhesion whereas more than 10 parts would adversely affect cure.
In order to apply the primer composition as a thin uniform coating, it is advantageous to blend an organic solvent in the primer composition. The type and amount of the organic solvent are determined by taking into account such factors as the ease of application of the resulting composition. Examples of the organic solvent include toluene, xylene, benzene, heptane, hexane, trichloroethylene, perchloroethylene, methylene chloride, ethyl acetate, and solvent gasoline. These solvents may be used alone or in admixture of two or more depending on the wetting of film backings with the resulting primer composition.
In applying the primer composition using the organic solvent, an alkenyl group-bearing polyorganosiloxane having a degree of polymerization of at least 3,000 may be added to the primer composition for further improving coating characteristics insofar as it does not compromise the benefits of the invention. When the alkenyl group-bearing polysiloxane is added, the amount of component (B) is increased if necessary.
Other optional components which can be added to the primer composition include well-known additives, for example, heat resistance improvers, coloring agents and antistatic agents, such as red iron oxide, cerium oxide, cerium hydroxide, titanium oxide, and carbon black. Such additives are included insofar as they do not compromise the benefits of the invention.
On use, the primer composition is prepared by uniformly mixing the above-mentioned components, then uniformly applied onto a plastic film, followed by heat curing. The coverage or buildup of the primer composition is a sufficient amount to form a thin primer film on the plastic film surface, for example, about 0.05 to 2.0 g/m2. Application of an excessive amount rather impedes the adhesion of the silicone pressure-sensitive adhesive to the backing and is thus undesirable. After application, the primer coating is heated for curing. The temperature for heat curing may be selected as appropriate in the range from 80xc2x0 C. for 120 seconds to 180xc2x0 C. for 15 seconds although the curing conditions vary with the type of film backing and the coverage.
The plastic film backings to which the primer composition is applicable include polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyimide resins, polyimide ether resins, epoxy resins, phenolic resins, polyphenylene sulfide resins, ABS resins, and perfluoroalkoxy (PFA) resins.
The silicone pressure-sensitive adhesives which can be applied onto the primer include well-known adhesives, for example, adhesives of the peroxide curing type to be cured with organic peroxides, and adhesives of the addition curing type to be cured in the presence of platinum base catalysts at room temperature or elevated temperature. It is noteworthy and advantageous that the primer composition of the invention is compatible with silicone pressure-sensitive adhesives of the addition curing type.
The primer composition of the invention helps a silicone pressure-sensitive adhesive form a firm bond to a plastic film backing. The firm bond of the primer composition is fully durable for a time, that is, aged adhesion is good. The primer composition thus finds a wide variety of applications.